As for the jobs like Dragoon not levelling up past 100 because they feel they have maxxed them out, it can be done but players still want to feel like they are learning new abilities. One way to do is to do something like Covenants from WoW, where once the player has become the best paladin they can be they then start learning a new skillset, like learning Night Fae magicks, or like how in GW2 once the player reaches max level on Ranger they then go on to learn new skillsets in the expansions like becoming a Soulbeast or an Untamed. Though this would mean reviving FF11's design philosophy of being able to multiclass, ie you are a Samurai/Beastmaster or Night Fae Paladin.

If they want to keep all of the cities populated, then they need to implement GW2's megaserver system where it dynamically phases every city to keep them populated at all times. Put every 5 players hanging out in Kugane on every server into the same instance so now you have 200 people in Kugane.

24 man raiding sounds neat for people who want to go through the same scripted corridor instance over and over again.